In the early years of multilayer printed circuit board manufacturing, lamination tooling holes were optically positioned in relation to predetermined target locations and then drilled or punched through the post-etched material. In most cases, the method was inaccurate, tedious and slow, but the process had some conceptual advantages not found in the pre-image process largely used today. Since fewer steps in artwork preparation were necessary, tolerance errors were reduced. Also, artwork registration could be done without pins, making imaging a faster process.
Problems arose, however, because the inherent instability of the etched material caused a deviation in target locations. When holes were punched or drilled in the target, this deviation caused the tooling holes to vary in relationship to each other. The error then compounded from layer to layer, causing the material to buckle during lay-up.
With the introduction of newly developed equipment, all the advantages of post-etch punching can be realized while the disadvantages are overcome. One such system is seen in U.S. Pat. No. 4,568,971. This system punch automatically averages any deviation in the target locations due to material instability, splits the difference, and punches tooling slots or holes into the panel.
The dimensional instability of epoxy glass laminates means registration at any stage in the process is never perfect because the base material is either shrinking or growing. The best method of correction after accumulating various tolerance errors is to "split-the-difference." This is achieved most effectively by implementing the four-slot tooling method commonly used for multilayer tooling. A good tooling system allows the multilayer manufacturer to compensate for any error in the artwork or laminate before tooling slots or holes are punched.
U.S. Pat. No. 4,568,971 (D. Alzmann et al) discloses a method and apparatus for effecting operations such as punching holes in sheets of material at precise locations. Each sheet has a reference marking thereon and is placed on a receiver in sequence and held in fixed position thereupon. A TV camera views each sheet to detect the reference marking thereon and a visual display of the marking is produced on a TV monitor. Also produced on the monitor is a cross hair with a coincident shaded box. The cross hair is first aligned with a marking reference on a master sheet to establish a fixed relation between the cross hair and the marking reference on the master sheet and thereby between the cross hair and the operating system which may be, for example, the punches which punch the holes in the sheets. The master sheet is removed and the sheets to be punched are placed one by one on the receiver which is moved to align the reference marking on each sheet as viewed on the monitor.